Conventionally, in a ball joint provided in various devices of a vehicle or the like, a dust cover is used in order to prevent entry of water, dust, etc. into a joint portion and prevent an outflow of grease from the joint portion. The dust cover used in the ball joint according to a prior art will be described with reference to FIG. 14. FIG. 14 is a schematic cross-sectional view of the ball joint and the dust cover mounted to the ball joint according to the prior art.
The ball joint includes a ball stud 300 that has a spherical portion at one end of a shaft portion, a socket 200 that rotatably and swingably supports the ball stud 300, and a knuckle 400 that is fixed to an opposite side of the shaft portion to the spherical portion. A dust cover 700 includes a seal main body made of an elastic body and integrally having a deformable film-like body portion 710, a fixed portion 730 that is provided on one end side of the body portion 710 and is fixed to the socket 200, and a seal portion 720 that is provided on the other end side of the body portion 710. Herein, the seal portion 720 includes an inner peripheral seal portion 721 that is provided so as to be slidable on the shaft portion in the ball stud 300, and a dust seal portion 722 that is provided so as to be slidable on the knuckle 400.
In addition, in the seal portion 720, a reinforcing ring 750 made of metal or resin is buried. Thus, by burying the reinforcing ring 750 in the seal portion 720, it becomes possible to improve sealing performance by the inner peripheral seal portion 721 and the dust seal portion 722.
Herein, for example, as shown in FIG. 14, in a state in which the ball stud 300 is tilted to the left in the drawing, the left part of the body portion 710 in the drawing is compressed, and the right part thereof in the drawing is extended. With this, in the right part in the drawing, a force acts on the inner peripheral seal portion 721 such that the inner peripheral seal portion 721 is caused to move away from the shaft portion in the ball stud 300, and a force acts on the dust seal portion 722 such that the dust seal portion 722 is caused to move away from the knuckle 400. As described above, by burying the reinforcing ring 750 in the seal portion 720, the inner peripheral seal portion 721 is prevented from moving away from the shaft portion, and the dust seal portion 722 is prevented from moving away from the knuckle 400.
However, in a low temperature environment, elasticity of each of the inner peripheral seal portion 721 and the dust seal portion 722 that are made of rubber is significantly lost. In addition, with a permanent set resulting from compressive deformation at a low temperature, gaps may be formed between the inner peripheral seal portion 721 and the shaft portion in the ball stud 300 and between the dust seal portion 722 and the knuckle 400. With this, a sealing function may be reduced.
Note that it is possible to more reliably prevent the inner peripheral seal portion 721 from moving away from the shaft portion in the ball stud 300 by reducing the inner diameter of the reinforcing ring 750. However, as the inner diameter of the reinforcing ring 750 is reduced, a pressing force to the shaft portion by the inner peripheral seal portion 721 is increased correspondingly, and it becomes difficult for the inner peripheral seal portion 721 to slide on the shaft portion. With this, there arises a new problem that the film-like body portion 710 is twisted. Accordingly, the measure by reducing the inner diameter of the reinforcing ring 750 has limitations.